1. Field of the Invention
The present invention relates to a method of measuring the surface form of a semiconductor thin film and, more specifically, it relates to a method of measuring the surface form of a polysilicon film formed on a semiconductor substrate to constitute a capacitor electrode.
2. Description of the Related Art
A semiconductor memory element such as a DRAM (dynamic random access memory) is provided with numerous capacitors. Capacitors formed at such a memory element are formed by laminating a lower electrode, a dielectric film and an upper electrode on a semiconductor substrate. In recent years, the application of the technology whereby the capacities of the capacitors formed at memory elements are increased by forming indentations and projections at the surface of the capacitor electrode constituted of a polysilicon film facing opposite the dielectric film and, therefore, increasing the surface area of the dielectric film has become common.
Since the charge storage capacity of a capacitor is affected by the degree/height of the indentations and projections formed at the surface of the capacitor electrode, consistency must be achieved in the degree/height of the indentations and projections formed at the surface of the capacitor electrode in order to achieve consistency in the charge storage capacity of the capacitor. For this reason, the surface form of the capacitor electrode must be measured.
Instead of directly measuring the surface form of the capacitor electrode, a method in which the thickness of the capacitor electrode, i.e., the film thickness of the polysilicon film, is measured is adopted in the prior art. Since the surface form of the capacitor electrode cannot be directly observed on an AFM (atomic force microscope), a direct examination is always implemented by utilizing an SEM (scanning electron microscope).
The surface form of the capacitor electrode, which has a close correlation to the capacitor capacity is one of the most crucial production management items and must be closely controlled in order to provide high-quality memory elements. If an abnormality is found in polysilicon film thickness measurement during a memory element manufacturing stage, it should be assumed that the surface form of the capacitor electrode does not meet the specifications. Accordingly, the conditions under which the polysilicon film is formed must be reassessed. However, it is not possible to accurately ascertain the surface form of the capacitor electrode in correspondence to the film thickness of the polysilicon film. Thus, it is difficult to promptly determine the optimal film forming conditions under which the polysilicon film should be formed in order to achieve the desired surface form for the capacitor electrode.
Observing the surface form of the capacitor electrode with an SEM, on the other hand, is a laborious process, requiring a great deal of time. As a result, it reduces the productivity in semiconductor element production.
An object of the present invention is to provide a method of measuring the surface form of a semiconductor thin film such as a polysilicon film quickly and easily.
Accordingly, the present invention provides a method of measuring the surface form of a semiconductor thin film formed at a surface of a semiconductor substrate. The method, which comprises a step in which a graph representing the relationship between tan xcfx86 and the measurement wavelength and a graph representing the relationship between cos xcex94 and the measurement wavelength with regard to the semiconductor thin film are prepared through spectro-ellipsometry, a step in which the film thickness of the semiconductor thin film is optimized by optimizing a coefficient in a Cauchy model function on the premise that five layers of film are laminated on the surface of the semiconductor substrate with film thicknesses T1, T2, T3 and T4 and void rates V1, V2, V3 and V4 respectively corresponding to a first layer, a second layer, a third layer and a fourth layer of the semiconductor thin film starting from the side toward the semiconductor thin film surface among the five layers are ascertained by fitting the optimized film thickness in the graph representing the relationship between tan xcfx86 and the measurement wavelength and the graph representing the relationship between cos xcex94 and the measurement wavelength and a step in which the values thus ascertained are used for substitution in the following expression (1) to calculate a total void rate X:
X=(T1xc3x97V1+T2xc3x97V2+T3xc3x97V3)/(T1+T2+T3)xe2x80x83xe2x80x83(1)
is characterized in that the surface form of the semiconductor thin film is measured based upon a correlation between the total void rate X and the surface form of the semiconductor thin film.
According to the present invention, the surface form of the semiconductor thin film may refer to, for instance, the surface area of the semiconductor thin film, and in the case of a memory element having numerous capacitors such as a DRAM (dynamic random access memory), it represents an index of the capacitor capacity, i.e., an index of the effective film thickness.
In addition, the present invention provides a method of measuring the surface form of a semiconductor thin film, which is achieved by uniformly depositing IPA on the surface of the semiconductor thin film formed at a surface of a semiconductor substrate, performing a quantitative analysis of IPA released from the surface of the semiconductor thin film and measuring the surface form of the semiconductor thin film based upon a correlation between the quantity of IPA thus detected and the surface form of the semiconductor thin film.